Date: Tue, 27 Aug 2013 14:46:21 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Lee Jones <lee.jones@linaro.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>,
        Linus Walleij <linus.walleij@linaro.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>
Subject: Re: [PATCH 09/33] ARM: ux500: Supply the I2C clocks lookup to the
 DBX500 DT
Message-ID: <20130827134621.GJ19893@e106331-lin.cambridge.arm.com>
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On Tue, Aug 27, 2013 at 09:06:35AM +0100, Lee Jones wrote:
> On Fri, 23 Aug 2013, Mark Rutland wrote:
> 
> > On Fri, Aug 23, 2013 at 08:56:07AM +0100, Lee Jones wrote:
> > > I had a short chat with Rob last night about this. I'm going to loop
> > > him in to the conversation, as he wrote the binding.
> > > 
> > > > > When most of the other clocks that we deal with are being requested,
> > > > > they rely on being index zero:
> > > > > 
> > > > >   drivers/i2c/busses/i2c-nomadik.c: dev->clk = clk_get(&adev->dev, NULL);
> > > > 
> > > > Look at drivers/clk/clkdev.c, there's some fuzzy matching
> > > > involved when you pass NULL as connection id.
> > > 
> > > Yes, I've been looking at that. This is why it works currently. I
> > > think I need to change all of the drivers to specify which clock they
> > > want. At the moment that 'fuzzy matching' is what's saving us. If
> > > anyone were to change our DTS file to match what the binding says,
> > > then it would cease to work. I'm guessing this is the same for all
> > > other DTS files too:
> > 
> > I think if anything, the binding document(s) should be updated to
> > describe that apb_pclk is referred to by name, and the names of the
> > other clocks should be described in the specific device bindings. We can
> > then modify the drivers which grab clock 0 to explicitly grab the first
> > clock by name, and backwards compatibility should not be broken.
> > 
> > I don't believe any other OS has implemented the common clock bindings,
> > and we've never supported the binding as described. Let's correct the
> > de-facto standard into a standard by decree.
> 
> I think we need to respect, or at least take into consideration the
> reason for the original 'de-facto' standard. Other OSes shouldn't be
> forced to provide a named clock request in order to obtain
> 'apb_pclk'. If the binding says it should be first, then perhaps we
> should do just that. It's simply a matter of naming all subsequent
> clocks related to AMBA devices.

Ideally, yes. However, we have to be careful to not break compatibility.

I took a look at existing primecell drivers and what they do. The
situation isn't so bad (with the exception of the
half-dt/half-platform-code mess):

* Some don't deal with clocks at all (no clk* in the driver). pl320 is
  in the ecx-common dtsi with only apb_pclk but has no binding
  defined. Some have no clocks defined in the dt and are presumably few
  clocks by platform data or are non-functional.

  I'm not sure how these DTs are going to be supported if and when we
  remove the platform data they depend upon. If we're really going to do
  that, then they are clearly not supported as-is long term.

* The pl022 driver grabs the first clock to figure out the rate of the
  spi bus (assuming it is SSPCLK). The SSPCLK input is not defined in
  the binding. The ste-u300 dts has two clock-names, "apb_pclk" and
  "spi_clk" (in that order), but they refer to the same clock.

  Given the existing driver simply grabs the first clock and they're
  both the same, we could re-order the names and make the driver grab
  the second clock. That wouldn't break backwards compatibility with the
  sole dts file we have using the binding, though this assumes no-one
  else has a dt lying around with different clocks.

* pl010 grabs the first clock given to it to figure out the uart rate
  (assuming it is UARTCLK), but it's only in integratorap.dts, without
  clocks, and is presumably fed by platform data. There is no binding
  document.

  pl011 grabs the first clock given to figure out the UART rate
  (assuming it is UARTCLK). The binding explicitly states it's only
  given apb_pclk, despite UARTCLK and PCLK being separate inputs to the
  IP block.

  These two bindings don't describe the hardware, and should be fixed.
  The only way I can think to make this work without breaknig backwards
  compatibility would be to try to grab the second clock and then fall
  back to the first if there isn't one. The other option is to break
  backwards compatibility, but I'm not sure that's much of an option.

* pl111 has no driver or binding in mainline, but appears in dts files.
  Those dts files clcdclk and apb_pclk, in that order. We could fix
  those before a driver starts using them.

If you think those suggestions are OK, I can put together a series to
fix this.

Thanks,
Mark.
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